The proposed work will deal with three subjects: (1) The structure of closed circular denatured DNA prepared from DNAs with different superhelix densities. The study will be performed by sedimentation methods as a function of temperature and the denaturing character of the solvent. (2) A restriction enzyme fragment map of mitochondrial DNA will be assembled, so as to position the sites of ribonucleotides that occur in this DNA and also to position the various RNAs formed by in vivo transcription of this DNA. (3) Centrifugal procedures for segregating animal cells growing in exponential suspension cultures are being refined so as to have material available for biochemical studies associated with DNA metabolism events that occur in the mitochondria as a function of the cell cycle.